Answer:
Feet/year
Note: The growth of a tree takes the form of a change in height
The unit for measuring a change in height can be feet, meters, centimeters, inches, etc.
The rate of change = (Change in height) / Time taken
The unit of time can be seconds, minutes, hours, day, months, year.
Therefore, in expressing the unit for the rate of chnage of a tree, the numerator has to be an acceptable unit of height and the denominator, a unit of time.
Option A ( Feet/year) is correct because feet is a unit of change in height and year is a unit of time
Option B(Miles per gallon) is wrong because, though miles can be a unit of height, gallon cannot be a unit of time
Option C (Centimeters/month) is correct because centimeters can be a unit of height and month can also be a a unit of time
Option D (Litres/day) is wrong because, despite that "day" can be a unit of time, "Liters" can never be a unit of height